It is well known that an aromatic polyimide dense film or asymmetric membrane has utility in separating gases. Such polyimide membranes have an imide linkage in the polymer backbone. They are taught in U.S. Patent Re. 30,351, which is incorporated herein by reference. U.S. Pat. No. 4,705,540; U.S. Pat. No. 4,717,394 and co-pending application Ser. No. 07/418,838, which are incorporated herein by reference, disclose preferred polyimide membranes.
The polyimide membranes can be produced by casting films or extruding hollow fibers from dope solution of aromatic polyimide or corresponding polyamic acid precursors in a solvent mixture and evaporating the solvent (or, in the case of the asymmetric membrane, evaporating a portion of the solvent from one side of the film or the hollow fiber and quenching with a nonsolvent). Such processes are taught in the above references as well as U.S. Pat. Nos. 4,370,290; 4,474,662; 4,485,056; and 4,528,004, which are incorporated by reference. In the case of the polyamic acid, cyclizing by a suitable treatment such as the application of heat is needed.
The preferred asymmetric membrane is characterized by a thin skin of polymer supported by a generally cellular structure. The thin skin is preferred since it allows high flux or permeation of gases. Forming a highly permeable membrane also leads to formation of submicroscopic holes which pass gases indiscriminately causing the membrane to have an effective separation value for most pairs of gases which is less than the intrinsic separation value of the polymer from which the membrane is made. Posttreating is typically used to heal the holes.
U.S. Pat. No. 4,370,290 discloses an aromatic polyimide porous fiber produced from a spin dope solution of a polymer material consisting of at least one polyimide in a solvent comprising, as a main component, at least one phenolic compound.
U.S. Pat. No. 4,474,662 discloses an aromatic polyimide porous membrane produced by drying a thin layer of dope solution containing an aromatic polyamic acid in a mixed solvent of polar organic compounds which evaporate at different rates and in which the polyamic acid is soluble to different extents.
U.S. Pat. No. 4,485,056 discloses an aromatic polyimide membrane produced by forming a thin film of a dope solution containing aromatic polyamic acid in a polar organic solvent, coagulating the thin dope solution film in a coagulating liquid containing water and lower aliphatic alcohols, drying the coagulated membrane and heating it to convert it to an aromatic polyimide membrane.
U.S. Pat. No. 4,528,004 discloses an aromatic polyimide porous membrane substrate with at least one surface coated from a dilute solution of aromatic polyimide or polyamic acid dissolved in one or more organic liquids. Prior to applying the dope solution, the porous membrane may be immersed in a treating liquid of at least one member selected from lower aliphatic alcohols, lower aliphatic ketones, benzene-type liquid compounds.
Co-pending patent application Ser. No. 07/418,838 discloses aromatic polyimide membranes compositionally containing phenylindane functions.
It would be desirable, particularly in the case of asymmetric membranes, if processing improvements could be made and if a membrane with fewer defects could be made prior to posttreatment.